This invention relates generally to multiplexing systems, and, more particularly, to an improved optical multiplexer or demultiplexer.
In practice the need frequently arises for communications or reconnaissance systems which simultaneously convey multiple messages from a large number of information sources in one location to a large number of users at another location. Multiplexing systems economically meet this need by combining the messages from several information sources, which are then transmitted as a composite group over a single transmission facility, with provision at the receiver for separation (demultiplexing) back to the individual messages. Since only one transmitter and receiver is needed instead of many, one advantage of multichannel operation is a lessening of the total quantity of necessary equipment. Each of the individual streams of information that form a multiplexed group are often denoted as a channel.
There are generally two generic forms of multiplexing systems. These are frequency division multiplexing and time division multiplexing. Frequency division multiplexing is directly applicable to continuous wave form (analog) sources, in essence involving (stacking) side-by-side in frequency several information channels so as to form a composite signal. The composite frequency-multiplexed signal is then used to modulate a carrier in some conventional manner. Recovery of the individual messages after reception and initial demodulation is accomplished by bandpass filtering and frequency selection of the channels.
Time division multiplexing is a logical extension of pulse modulation, and involves interleaving in time the narrow pulses of several ordinary pulse modulation signals and thus form one composite pulse transmission system. Separation of the time multiplexed pulse streams at the receiver is accomplished by gating appropriate pulses into individual channel filters.
A third technique, phase multiplexing, is possible but appears less practical than either frequency division multiplexing or time division multiplexing. Unfortunately, the multiplexing systems of the past have been deficient in the area of low loss optical multiplexing or demultiplexing of more than two closely spaced optical frequencies.